1. Field of the Invention
The present invention relates to a steel sheet for use in the manufacture of an end for a so-called easy-to-open can, the steel sheet having an improved property which renders the can easy to open. The present invention also relates to a process for producing such steel sheet and to an end for an easy-to-open can.
2. Description of the Prior Art
In recently widely used easy-to-open cans, such as beer cans, carbonated beverage cans, juice cans, etc., the end of the closed vessel is engraved, i.e., is provided with a score, and a tab is secured on a portion of the end within the score. By pulling off the tab, the score is torn and the can is opened. As the material of the end for easy-to-open cans, i.e., the easy-to-open end, aluminum is, conventionally, more frequently used than are steels since less strength is required to open an aluminum can than a steel can, thereby rendering the can-opening process easier than in the case of steel cans. However, the production of aluminum cans is more expensive than the production of steel cans, and aluminum cannot be used for all types of containers because of its poor corrosion resistance in the case of certain types of contents. In addition, the entire can is desirably made of the same material, e.g., steels, in the light of the recycling of empty cans. Accordingly, there have been strong demands for the development of a steel sheet which exhibits a property rendering a can easy to open which is approximately equal to the easy-to-open rendering property of aluminum.
Generally speaking, since the can-opening property, i.e., the ease with which a can can be opened, depends upon the residual thickness after score working, i.e., the thickness of a steel sheet between the bottom of the score which has an inversed pedestal shape and the rear surface of the steel sheet, the can-opening force is decreased with a decrease in the residual thickness after score working. If, however, the residual thickness after score working is very small, cracks are liable to form during the score working, and, in addition, the score is occasionally damaged during handling of and transportation of the easy-to-open cans. The residual thickness after score working, at which the score can be formed without the formation of cracks, is referred to as the critical residual score thickness and is increased with an increase in the thickness and strength of a steel sheet and the number of coarse intermetallic inclusions. The usual residual thickness after score working is the critical residual score thickness plus a safety margin thickness of from 10 .mu.m to 20 .mu.m. The critical residual score thickness is relatively small, for example, from 60 .mu.m to 70 .mu.m, with regard to non-inner pressure cans for which a thin steel sheet having a thickness of approximately 0.23 mm can be used. With regard to inner pressure cans, a relatively thick steel sheet, e.g., an approximately 0.29 mm steel sheet having a T-4 temper, must be used, and, thus, the critical residual score thickness must be approximately 90 .mu.m or less. In inner pressure cans made of steels, the inferiority of the can-opening property thereof to that of inner pressure cans made of aluminum is greater than that of non-inner pressure cans. In addition, since the steel sheet used for inner pressure cans is thick, the advantage of steels in the light of cost is small. Thus, steel ends for easy-to-open cans are not at all being put to practical use.
Japanese Unexamined Patent Publication No. 57-198,244 discloses that in a steel sheet for use in the manufacture of DI (drawn and ironed) cans the flange working property is improved by refining the size of the cementite particles. The DI cans need not, however, be subjected to score working and rivetting to secure a tab thereon.
U.S. Pat. No. 3,777,536 discloses a steel sheet for use in the manufacture of easy-to-open cans, in which steel sheet phosphorus is incorporated to embrittle and to thus improve the can-opening property thereof.
It is also known to incorporate tin, as an embrittling element, into steels or to coarsen the cementite in the steels, thereby improving the can-opening property. In order to form an end for an easy-to-open can, the end must be rolled around the drum, and a tab must be secured on the end by means of rivetting.
The known methods for improving the can-opening property seriously deteriorate the ductility and, thus, the end-forming property.